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Moon shadows signal approach of Saturn equinox

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 24 March, 2009

For the first time, Cassini has captured the shadows of the planet’s moons on Saturn’s broad expanse of rings as the majestic planet approaches equinox.

Just like the Earth, Saturn’s spin axis is tilted relative to its motion around the Sun. This means that from the view point of Saturn, the Sun makes a cyclical passage across the Saturnian sky, sweeping from the southern hemisphere to the north and back again. One full cycle of Saturn’s season is equivalent to 29.5 Earth years, meaning that the Sun passes through the plane containing the planet’s rings roughly every 15 Earth years.

During this celestial alignment, the shadows of the planet’s rings fall in the equatorial region on the planet, and the shadows of Saturn’s moons external to the rings, especially those whose orbits are inclined with respect to the equator, begin to intersect the planet’s rings. Any vertical protuberances within the rings, including small embedded moons and narrow vertical warps in the rings will also cast shadows on the rings. At exactly the moment of equinox, the shadows of the rings on the planet will be confined to a thin line around Saturn’s equator and the rings themselves will go dark, being illuminated only on their edge.

Named for these unique illumination circumstances, the extended Cassini Equinox Mission is gathering observations on the planetary line-up, beginning with the appearance of the shadows of Saturn’s moons on the icy platform of its rings. The images will provide valuable information regarding the presence of any deviations across the rings from a perfectly flat wafer-like disc.

The main rings - which, working out from the planet, are named C, B and A - are only around 10 metres thick, and lie inside the F ring, which is vertically thicker than the A, B and C rings, making the determination of interior vertical deviations difficult when imaging the rings edge-on. “We hope that such images will help us measure any vertical warping in the A and B rings,” says John Weiss, an imaging team associate who planned the observations at the Cassini Imaging Central Laboratory for Operations (CICLOPS). “Because we know how big the moons are, and where they are in their orbits around Saturn when they cast these shadows, we have all the information we need to infer any substantial vertical structure that might be present.”

In the centre of this image, the shadow of Pan is a short streak thrown over the edge of the A ring where Pan travels its path through the Encke Gap. Image: NASA/JPL/Space Science Institute.

The first moon observed casting a shadow on the outer edge of the A ring was 113 kilometre wide Epimetheus, on 8 January. Next, tiny Pan, 30 kilometers across and orbiting within the rings, was caught casting a shadow on the A ring on 12 February. Eventually, more moons will cast shadows on the rings and all shadows will grow longer as exact equinox approaches. The shadows will be at their longest just before and after equinox when the Sun exactly crosses the ring plane on 11 August 2009.

“One of the best things about being in orbit around Saturn are those mind-expanding opportunities that arise every now and again to see some celestial phenomenon you couldn’t possibly see here on Earth,” says Carolyn Porco, leader of the Cassini imaging team. “It’s at those times you feel a real sense of privilege to be alive, now, to witness such remarkable sights. And from the looks of it, the next year is going to be one remarkable sight after another.”

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